This invention relates generally to cathode ray picture tubes, and is specifically addressed to an electron gun for use in cathode ray tubes used in projection television systems in conjunction with associated projection lenses.
Projection television systems typically include at least one cathode ray picture tube having a cathodoluminescent screen on the inside surface of the face panel. Electron-beam generating means disposed on the cathode ray tube electron-optical axis provide for forming an electron image on the cathodoluminescent screen. This electron image is converted to a visible image by the screen. Projection lens means on the projection optical axis of the tube provide for projecting the aerial image of the visible image onto a viewing screen whereon the viewer sees the television picture. The viewing screen may be of the front-projection type, or of the rear-projection type wherein the aerial image is projected onto the side of the screen opposite the viewer. To provide for compactness of the projection system, the path of the aerial image is normally "folded" by means of one or more mirrors.
Projection television systems may have a bank of red, green and blue image source means including three cathode ray picture tubes each with an associated projection lens for projecting into coincidence a red, green, and blue image to form a composite color image on the viewing screen.
A desirable--indeed necessary--feature of a projection television system is the ability to project an image of adequate brightness on the viewing screen. Brightness preferably should be equal to that of the typical shadow mask color picture tube which provides an average brightness of 80 foot-Lamberts at a beam current of 1.5 milliamperes, and with a peak brightness potential of about 320 foot-Lamberts. In view of the relatively long projection path and consequent effect of the inverse-square law, this brightness objective has proved difficult to achieve in projection television systems.
The face of a cathode ray picture tube used in projection television systems can be circular, with a diameter of about six inches. Alternately, the face panel can be of rectangular configuration, with dimensions of approximately 4.5 inches in height, and 5.5 inches in width, by way of example. The visible image that is electron-formed on the cathodoluminescent screen on the inner surface of the face plate is a rectangle of three to four aspect ratio. To provide a projected image of four feet in diagonal measure having a brightness of eighty foot-Lamberts, for example, the brightness of the image onthe cathodoluminescent screen of the projection tube must be in the range of eight thousand to nine thousand foot-Lamberts.
Image brightness of this magnitude requires a relatively high beam current, typically about 1.5 milliamperes. High beam current in turn connotes a relatively large beam spot size on the imaging screen with consequent degradation of resolution. For example, attempting to increase image brightness by increasing beam current results in a loss of image resolution as beam diameter, and hence spot size, is in general proportional to the square root of the beam current. An increase in beam current from one milliampere to two milliamperes in a standard bipotential gun, for example, will normally increase spot size by about forty percent. And the larger the beam spot size, the lower the resolution. Hence an electron gun for use in a projection television cathode ray tube must be able to form a relatively small beam spot while operating at relatively high beam currents.
Electron guns used in television picture tubes generally consist of two basic parts: (1) an electron beam source, and (2) a lens for focusing the electron beam on the phosphor screen of the cathode ray tube. Most commercial focus lenses are electrostatic and consist of discrete, conductive, tubular elements which are arranged in sequence on an axis. These electrodes are supplied with predetermined voltages which establish the electrostatic focusing field. A main objective in designing an electron gun is to produce a small, symmetrical beam spot on the cathodoluminescent screen of the tube. The electron beam focus lens of the novel electron gun according to the present invention is classifiable as a bipotential, and consists of two electrodes. The first is the focus electrode and the second electrode is a beam accelerating electrode.
The present invention is disclosed in referent copending application Ser. No. 079,926. My invention was included in the application to indicate an environment for the '926 invention, which is directed to a half angle control electrode (see reference No. 138 in FIG. 10 of the '926 disclosure).